The Big Bang Theory

The Alien Parasite Hypothesis

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The Alien Parasite Hypothesis
Big Bang Theory episode
Season 4, Episode 10
December 9, 2010
Guest star(s)
Episode guide

"The Boyfriend Complexity"

"The Justice League Recombination"
Season guide

Season 3

Season 5

"The Alien Parasite Hypothesis" is the tenth episode of the fourth season of the American sitcom The Big Bang Theory. This episode first aired on December 9, 2010.[1]


Amy is aroused by Penny's ex-boyfriend; Zack Johnson, and is unfamiliar with the feeling. Sheldon thinks she has contracted an alien parasite. Meanwhile, Howard and Raj battle out who is the Hero and who is the Sidekick in their relationship.

Extended PlotEdit

Zack delivering menus
Girls' night out.
Howard and Raj getting ready to wrestle.

Amy begins to have feelings for Penny's ex-boyfriend Zack, whom she meets in a bar while on a night out with Penny and Bernadette. At her lab, Amy and Sheldon later discuss her symptoms (elevated heart rate, clammy palms, dry mouth and an involuntary "hoo!"). Amy insists that she is not going through menopause. Sheldon prefers the possibility that Amy has been infected by an alien parasite. Their final reluctant conclusion is that Amy is horny. Penny tells Sheldon there is something he could do about that (i.e. satisfy her sexual cravings) He completely misses her point and tries to set Amy up a date with Zack. These actions prove fruitless as Amy finds Zack as dumb as Penny does. Amy decides to use the Vulcan practice of kolinar to subdue her feelings even though she considers that cheap science fiction. She still lets out one more "hoo" before leaving Zack's presence. As Sheldon and Amy walk down the street, she grasps his hand. He asks her why she is doing this; she replies that it is an experiment. After a short interval, she lets go and says, "No, nothing. Never mind."

Meanwhile, Howard and Raj quarrel over who is the better superhero and sidekick between them. First they see who is braver my sticking their hand in a jar that contains a spider. Raj doesn't flinch because the spider was crawling up Howard's arm. Then they get Leonard to referee a wrestling match between them, but neither guy gets touched as they dance in circles for hours.


  • The TV Critic: "The writers continue to use Amy to deal with Sheldon plots which they realise are too valuable to use up right now. So instead of Sheldon feeling his first stirrings of sexuality we substitute Amy . I think it's a good idea in the sense that The Big Bang Theory is going to run for the foreseeable future so why blow through Sheldon stories when we can test them on surrogate Sheldon first. Sheldon actually turns out to be a good friend, going to great lengths to satisfy Amy's urges even if it means losing respect for her...The Amy-Sheldon relationship is still something I enjoy exploring even if the writers never figure out quite how to pitch her. The rest was poor filler." [2]
  • The A.V. Club gave this episode a C+.[3]
  • IMDb user reviews


  • Title Reference: The title refers to one of Sheldon's theories about why Amy had her symptoms.
  • Chuck Lorre's vanity card.
  • This episode was watched by 12.03 million people with a rating of 3.9 (adults 18-49).[4]
  • This episode aired in Canada on December 9, 2010 with 2.659 million viewers and a weekly ranking of #3.[5]
  • In England, this episode aired on January 6, 2011 with 1.192 millions total viewer and a weekly ranking of #17.[6]


  • Howard states, "I’m legally obligated to inform you that I took a karate lesson when I was 11. I’d be a regular ninja by now if my mom could’ve arranged a carpool." In "The Jiminy Conjecture," he similarly revealed, "Don’t push me, Sheldon. I may be small, but I took kung fu when I was 13, and I remember a good deal of it."
  • Rajesh and Howard argue superhero names Rat-Man, Mouse-Boy and Kid Vermin for themselves if one was bitten by a radioactive rat in a lab. Akin to this, in "The Hot Troll Deviation," Rajesh said if Sheldon's superpower were arrogance, his name would be Captain Arrogant, while Sheldon contended his name would be Dr. Arroganto.
  • This episode marks the first contest in a gym, with the second occurring in "The Rothman Disintegration."
  • Sheldon shows his favorite character Spock's salute (from Star Trek).
  • In "The Lunar Excitation," Amy said, "I don’t object to the concept of a deity, but I’m baffled by the notion of one that takes attendance." Amy suggests religion, in the episode of this article, when Sheldon says, "You know, in difficult moments like this, I often turn to a force greater than myself." This reaffirms she does not object to religious beliefs.
  • While speaking to Zack, Sheldon makes a reference to "The Justice League Recombination": "Good grief. It’s like trying to talk to a dolphin."
  • Sheldon and Amy's discussion of her physiological reactions (HOO!) to Zack is an homage to Abbott and Costello's famous "Who's on First?" routine.
  • This is the first time that, unlike Sheldon, Amy shows that she has sexual arousal.
  • "Lupus" on diagnosis board may refer to the TV series House, M.D.
  • Sheldon's favorite number 73 is coincidentally the episode number of this episode.
  • Amy's exposure to Penny has a greater effect on her and her sexually than Penny's effect on Sheldon. 
  • Rat-Man is the main character of an Italian comic book series.


Leonard: We get it! 73 is the Chuck Norris of numbers.
Sheldon: Chuck Norris wishes.

Howard: Let me ask you a question. What are you most afraid of?
Raj: I don't know. Nuclear war. Accidentally being buried alive. Any of those movies where you get a phone call that says you're going to die, and then you do.

Amy: HOO! (often repeated)

Amy: Penny's friend, Zack, stopped by and said "hello" and I said "hoo!"
Sheldon: Hoo?
Amy: Zack.
Sheldon: Then why did you ask?
Amy: Ask what?
Sheldon: Who?
Amy: Zack.
Sheldon: Alright, let's start over. What did you say when Zack walked in?
Amy: Hoo!
Sheldon: Zack.
Amy: Why do you keep saying 'Zack'?
Sheldon: Because you keep saying 'Who'.
Amy: I'm not saying 'hoo' now. I said 'hoo' last night.
Sheldon: And the answer was Zack, correct?
Amy: There was no question; I simply said "Hoo!"
Sheldon: [short pause] All right. I think I have enough to go on.

Amy: Sheldon, I am not going through menopause.


  • Sheldon leaving the bar with a Vulcan salute.
  • Figuring out what caused Amy's symptoms.
  • Zack runs into the girls.
  • Sheldon adding up Amy's symptoms.
  • Amy approaches Zack to propose coitus with him.
  • The spider is crawling up Howard's hand.
  • First girls' night out.
  • Raj and Howard planning to wrestle to prove a point.
  • Raj and Howard having a timid wrestling match.
  • Zack at the bar.
  • Working on Amy's illness symptoms.
  • Zack runs into the girls.
  • Howard trying to wrestle.
  • Amy experimenting with holding Sheldon's hand.
  • Gabbing in the laundry room.
  • Working on what's wrong with Amy based on her symptoms.
  • Chuck Lorre Productions, #319


  1. The Big Bang Theory - The Alien Parasite Hypothesis (TV Episode 2010) - IMDb
  2. The TV Critic's Review
  3. The Big Bang Theory: "The Alien Parasite Hypothesis" - TV Club - The A.V. Club
  4. Thursday Final Ratings: 'Bones,' 'Fringe,' 'Community,' '30 Rock,' 'Office,' 'Apprentice' Adjusted Down; 'Big Bang Theory,' 'Walters: Oprah' Up" - Zap2it
  5. Top Programs – Total Canada (English) December 6 – December 12, 2010 - BBM Canada
  6. BARB via Wikipedia
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